The primary function of the immune system relates to the protection of the body from disease. The immune system protects against not only those diseases which result from an attack by bacteria, viruses, and other pathogens, but also cancer, as well as disease states which result from immune imbalance, opportunistic infections, or autoimmune disorders.
Modulation of the immune system through pharmaceutically induced stimulation or suppression offers an important approach to the control of disease. Compounds which non-specifically stimulate the immune system are of potentially significant medicinal importance and have been the object of a lengthy research effort. Often, the research results show that immunomodulating compounds are either weak immunostimulants, and hence not very effective, or potent immunostimulants and, therefore, effective but toxic by virtue of this potent immunostimulating activity.
Among the many classes of compounds which non-specifically stimulate the immune system are nucleosides which are well known in the art. For example, 7-thia-8-oxoguanosine has been described by D. F. Smee et al. in the Journal of Biological Response Modifiers, 9, 24-32, 1990 as an antiviral agent in mice. The activity of this compound is derived from its ability to activate NK and B cells in the immune system, and to induce interferon. However, subsequent antiviral studies in humans as reported by P. G. Higgins et al. in Antiviral Chemistry and Chemotherapy, 2, 61-63, 1991, have disclosed few encouraging results. One problem has been the lack of oral bioavailability.
Other nucleosides have been synthesized and studied in an effort to develop an improved medication. For example, D. F. Smee et al. report in Antimicrobial Agents and Chemotherapy, 35, 152-157, 1991, that 7-deazaguanosine has significant immunostimulatory and antiviral activity after oral administration. However, these results are preliminary. With many nucleoside compounds, toxicity is an important issue which must also be closely analyzed.
A particular class of nucleoside immunostimulants has arisen from inosine and other similar hypoxanthine-containing compounds. A well know example is isoprinosine, an inosine-containing complex. Isoprinosine has been thoroughly studied as an immunomodulator and referred to as a "gold standard" by C. D. Simone et al. in Thymus, 19, 51-55, 1992. Some rationale for the activity of hypoxanthine-(inosine) containing compounds arises from the observation that a lack of adenosine deaminase, the enzyme which converts adenosine to inosine, results in severe combined immunodeficiency disease (SCID).
Although very nontoxic, isoprinosine is not an effective immunomodulator, and in order to improve its immunopharmacological properties, numerous analogues have been synthesized, as reported by J. W. Hadden et al. in International Journal of Immunopharmacology, 13, 49-54, 1991 (suppl. 1). In particular, they describe a prodrug in the form of inosine 5'-monophosphate (inosine, unless complexed, has little in vivo activity) and methyl inosine monophosphate (MIMP). However, MIMP is not a very active immunomodulator.
In an effort to retain the nontoxic properties of isoprinosine, but enhance the immunostimulatory activity, an immunomodulator was synthesized which contained both hypoxanthine and the amino acid L-arginine covalently linked by a pentamethylene bridge. The compound, ST 789 (hypoxanthine pentyloxycarbonyl L-arginine, formerly PCF 39) has been thoroughly described in a recent issue of Thymus, 19, S1-S112 (1992). L-Arginine was selected because it is known to play a role in immune activation and is present at the terminus of many immunomodulatory peptides such as tuftsin substance P, thymopentin, and splenopentin. ST 789 is further described in European Patent Application #91830284, publication #464,009, published Jan. 2, 1992. Analogues of ST 789 are also described in the European publication where oligopeptides composed of naturally occurring L-amino acids replace L-arginine. However, the purine base portion of the molecule remains hypoxanthine.
While no immunological comparison was made with isoprinosine, a similar pattern emerged. The compounds are nontoxic but, at best, moderate immunostimulants. For example, there was no indication that ST 789, or analogues thereof, could stimulate an important immune cell subset such as cytotoxic T lymphocytes (CD8.sup.+ T cells). This subset plays a key role in the defense of the body from viral infections and cancer.
P. Cornaglia-Ferraris describes still another analogue of ST 789 in International Journal of Immunopharmacology, 13, 1005-1012, 1991. In the published compound, L-arginine is replaced with the bombesin carboxy terminus dipeptide L-leucyl L-methionine. The purine base remains hypoxanthine. In fact, in this class of compounds where a purine base is covalently linked by a methylene chain to an amino acid or an oligopeptide, very little data has been reported for compounds including a purine base other than hypoxanthine. Further, because of the requirement for physiologically active amino acids in mammalian systems, all the work reported to date describes amino acids of the (natural) L-configuration. One brief description of the replacement of hypoxanthine with the naturally occurring purine bases adenine and guanine is reported by R. Stradi et al. in Fl. Farmaco, 45, 39-47, 1990, but there is no indication of significant biological activity.
As noted above, adenosine deaminase, and by implication inosine, is necessary to maintain normal immune status. Therefore, in U.S. Pat. No. 5,272,151 issued Dec. 21, 1993, M. Marzi et al. reported that in ST 789 the hypoxanthine is replaced with the xanthine oxidase inhibitor allopurinol. The result is ST 689, allopurinol pentanol. This substitution is expected to increase the concentration of inosine in vivo since inosine is catabolized to xanthine, and then uric acid in mammals in the presence of xanthine oxidase enzyme. However, allopurinol was noted to be immunosuppressive and ST 689 was not significantly more immunostimulatory than ST 789 in most of the immunology assays reported in the '151 patent.
Levamisole is another immunoregulator agent used against malignant melanoma. It has now been found that levamisole induces serious thrombocytopenia after starting adjuvant levamisole therapy for malignant melanoma [Med. Pediatr. Oncol. Apr 1995, 24 (4), 262-4].
The prior art indicates that there is a need for compounds which have the ability to stimulate a number of immune cell subsets and thereby possess significant immunomodulating activity, but, at the same time, lack toxicity.